For solving global environmental issues caused by exhaust gases, bodies of transportation machines such as automobiles should have lighter weights so as to improve fuel efficiencies. Accordingly, aluminum alloy materials have been increasingly used in automotive bodies instead of previously-used steel materials, because such aluminum alloy materials have lighter weights and are excellent in formability and paint bake hardenability.
Among them, AA 6000 or JIS 6000 series (hereinafter simply referred to as “6000 series”) Al—Mg—Si aluminum alloy sheets have been adopted as thin-thickness high-strength aluminum alloy sheets to panels such as outer panels and inner panels of automotive panel structures including hoods, fenders, doors, roofs, and trunk lids.
Such 6000 series aluminum alloy sheets basically essentially contain silicon and magnesium and have excellent age hardenability. When they are subjected to press forming or bending, they show lower yield strength and thereby have sufficient formability. In addition, they have bake hardenability (artificial age-hardenability or paint bake hardenability). Specifically, when they are heated at relatively low temperatures in artificial aging (hardening) such as paint baking of panels after forming, they undergo age hardening to have increased yield strength to thereby show sufficient strength.
The 6000 series aluminum alloy sheets contain relatively smaller amounts of alloy elements than those of, for example, 5000 series aluminum alloy sheets containing larger amounts of alloy elements such as magnesium. When the 6000 series aluminum alloy sheets are reused in the form of scraps as aluminum alloy melting materials (melting raw materials), ingots of 6000 series aluminum alloys can be easily obtained therefrom. Thus, they are also excellent in recyclability.
On the other hand, automotive outer panels are produced by subjecting aluminum alloy sheets to plural forming processes such as bulging and bending in press forming. In the formation of large-size outer panel structures such as hoods and doors, aluminum sheets are subjected to press forming such as bulging to yield formed articles as outer panels, and these outer panels are joined with inner panels to form panel structures by hemming such as flat hemming on the periphery of outer panels.
In this process, the 6000 series aluminum alloy sheets undergo natural aging. In particular, when they undergo natural aging for about three months to six months, they have markedly lowered paint bake hardenability and bendability due to increased yield strength and formation of atomic clusters.
For inhibiting deterioration in properties through natural aging and for improving room temperature stability, there have been made proposals to control atomic clusters, in particular, to control clusters of magnesium and silicon atoms which are formed when the aluminum alloy sheets are left at room temperature after solution heat treatment and quenching treatment.
For improving paint bake hardenability, for example, JP-A No. 2005-139537 relates to a technique of controlling a cooling rate in a solution heat treatment while focusing attention on a peak height in a differential thermal analysis curve. JP-A No. 10 (1998)-219382 and JP-A No. 2000-273567 relate to techniques for avoiding clusters of magnesium and silicon atoms (clusters of silicon and vacancy atom, Guinier-Preston 1 zone (GPI zone)). JP-A No. 2003-27170 relates to a technique of avoiding clusters of silicon and vacancy atom with respect to peaks in differential scanning calorimetry (DSC).